Viral proteins encoded in by 124 strain of Moloney murine sarcoma virus (MuSV) have been examined by immunoprecipitation with various antisera specific for gene products derived from non-defective murine leukemia virus. The immunoprecipitates were fractionated by electrophoresis in detergent-containing polyacrylamide gels. Radioactive viral polypeptides from the gels were further analyzed by peptide mapping procedures. Gene products of the three replication genes were detected in addition to a p10 deficient 'gag-src' fused proteins was isolated and characterized. In other experiments designed to generate antibodies against the MuSV 'src' gene product, rabbit cells transformed by the ml strain of Moloney MuSV were injected into young rabbits. Tumors developed at the site of inoculation within one week. The sera from such rabbits contained antibodies that recognized the 'env' precursor but not gp70, a p15(E)-like proteins and an approximately 55,000 dalton protein (p55) that is unrelated to any of the helper virus gene products. P55 appears to be ml MuSV specific since it was not found in normal cells (in significant amounts) or in murine leukemia virus-infected cells.